Wave energy converter

ABSTRACT

A wave energy conversion apparatus is described. It comprises at least two devices ( 3, 4 ), each comprising a surface float ( 5, 6 ), at least one of the surface floats being rigidly attached to a submerged body ( 9, 10 ). The movement of the two devices in response to a passing wave may be used to effect an energy transfer.

The invention relates to a point absorber wave energy converter,preferably comprising two or more heaving buoys floating on the surfaceeach rigidly linked to one or more deeply suspended vessels or bodies,the relative movement between the two bodies being utilised to generateenergy. The term wave or wave motion as used herein refers to both waveson a surface of a liquid and swell in a body of liquid

BACKGROUND

Quest for Economic Sources of Renewable Energy

The development of a practical wave energy converter has been the focusof attention from a number of engineers and theoreticians over the pasttwenty five years. Theoretical understanding of sea waves and technicalexpertise in related marine engineering has gained immeasurably from theoffshore oil and gas industries during the same period. Growing concernwith global climate change has led to an increased sense of urgency inthe quest for commercially viable renewable energy sources.

The Size of the Wave Energy Resource

The potential of wave energy has been recognised for many years. Thesize of this resource has been estimated to be 219 gigawatts along thecoasts of the European Union, or more than 180 terawatt hours each year.The Wave power off the west coasts of Ireland and Scotland, where thewinter resource is approximately twice that available during summermonths, ranks with the highest levels in the World.

The Offshore Resource is Greater

Wave energy is lost by friction with the sea bottom as the sea becomesshallow (water depths of half a wavelength or less). This is mostpronounced where wavelengths tend to be long, as off the NW coast ofEurope. On or close to the shore the availability of this alreadyattenuated resource is greatly diminished by the lack of physicallysuitable sites and restrictions imposed by planning controls.

Development of Wave Energy Converters.

Research and development into wave energy converters (WECs) over thepast twenty-five years, plus the knowledge and practical experiencegained from the offshore oil and gas industries, has now reached a stagewhere robust and effective wave energy converters with installedcapacities of one megawatt and greater are being developed.

Categories of Wave Energy Converters

The wave energy resource may be split into three broad categories, basedon where the energy from waves may be recovered:

-   1. in the open sea, i.e. offshore-   2. on or close to the shore line, i.e. on-shore or inshore-   3. outside the normal area of breaking waves but not in the deep    ocean, i.e. near shore.

The very large number of devices and concepts proposed to date has beenclassified and described in summary form for the Engineering Committeeon Oceanic Resources by the Working Group on Wave Energy Conversion(ECOR draft report, April 1998). This follows a similar classificationbased on the intended location, i.e. offshore, near shore to offshore,and on-shore.

Wave Energy Converters (WECs) may also be classified in different waysaccording to their operating principle and the ways in which they reactwith waves. In terms of practical application, only a very few types ofdevice are presently, or in the recent past have been, in use or undertest.

A significant fraction of the present generation of WEC devicesincorporate an Oscillating Water Column (OWC). OWC devices are typicallythose where the wave is confined in a vertical tube or a larger chamberand, as it surges back and forth, drives air through a power conversiondevice typically an air-turbine. Megawatt-scale OWC devices are now atan advanced stage of development. One such device, built in a rockygully on the western shore of Pico in the Azores, is a reinforcedconcrete chamber partly open at one side below the waterline to theaction of the waves. A similar but slightly smaller device, the LIMPET,has been installed on the cliff face of Islay in Scotland. These twoinstallations would seem to be the best-developed and perfected WECsystems of this size currently available. It is, however, unlikely thatany one such installation will have an installed capacity greater thantwo megawatts and the number of suitable sites has to be extremelylimited.

The present invention relates to an apparatus that may be of at least acomparable size, and capable of being deployed offshore and in largearrays. It is of a class of WEC's known as Point Absorbers.

Point Absorbers

Point absorbers are usually axi-symmetric about a vertical axis, and bydefinition their dimensions are small with respect to the wavelength ofthe predominant wave. The devices usually operate in a vertical mode,often referred to as ‘heave’. Typically, a surface-piercing float risesand falls with the passing waves and reacts against the seabed or a tautmooring. As such they are capable of absorbing energy arising fromchanges in the surface level rather than from forward motion of breakingseas. The theoretical limit for the energy that can be absorbed by asingle isolated, heaving, axi-symmetrical point absorber has been shownto depend on the wavelength of the incident waves rather than the crosssectional area of the device, i.e. from the wavelength divided by 2π.Thus the wavelength is a critically important criterion, resulting inthe attraction of locating the point absorber devices well outside theregion of breaking waves, and where they will be open to long wavelengthocean swell or ‘heave’.

Point absorbers may react against the seabed (therefore necessarilysited in relatively shallow water, usually near-shore), or be floatingand react against the inherent inertia of one of its components.

Small-scale practical point absorbers such as fog horns and navigationbuoys, both of which may incorporate OWCs, have been in use for manyyears. Typically these have a power of a few hundred watts.

Self-Reacting Heaving Buoy Point Absorbers.

There have been several attempts to develop wave energy converters basedon the self-reacting heaving buoy principle. One such example is aheaving buoy which reacts against an inertial plate suspended below.This concept has been described and analysed by Berggren, L. andJohansson, M., Hydrodynamic coefficients of a wave energy deviceconsisting of a buoy and a submerged plate. Applied Ocean Research,0141-1187/92/05.00 and by Falnes, J., Wave-energy conversion throughrelative motion between two single-mode oscillating bodies (OMAE,Lisbon, Portugal, Jul. 5-9, 1998).

A second variation of the heaving buoy principle is described in aninternational patent application, WO 97/41349. In this, a single heavingbuoy reacts against a column of water trapped in a cylinder suspendedvertically below and open at either end, by means of a wide pistonmoving reciprocally within the cylinder. The column of water moved bythe piston acts as an inertial mass; this arrangement is known as anaccelerator tube. Similar technology is known and described in U.S. Pat.No. 4,773,221.

In these illustrative examples and all such self-reacting heaving buoysystems, there are essentially three basic components: a heaving buoy onthe surface, some form of reaction device suspended below (an inertialplate, accelerator tube, etc.) and a load resistance or power take-offplaced between them.

Latching and Phase Control

It is also known to use a principle of latching the phase control of aheavy body The principle of latching a heaving (vertically oscillating)body in irregular waves having been described by Budal and Falnes in1978 British Patent No. GB 1587344.

Their idea was to force the phase of a heaving float to follow that ofthe waves, which had a significantly lower natural frequency (longerperiod). In this way greatly amplified motions and correspondinglylarger power levels were achieved.

They disclose the holding of the heaving body at the top or bottom ofits cycle by a hydraulically operated latching mechanism (functioning asa parking brake), locking the heaving float to a long rod attached tothe bottom of the wave channel. It was then released so that it wouldresume motion in direction and in phase with the wave. Furthertheoretical analysis has been completed by various researchers. Twoforms of such ‘phase control’ are now recognised, i.e. latching asdescribed and continuous control which may be applied throughout thecycle and may involve power being returned to the heaving device.

Variable Buoyancy Apparatus

A further development in self-reacting point absorbers incorporates athree-body point absorber comprising a surface float, a submergedvariable buoyancy and an inertial mass. Such a device is known anddescribed in our corresponding international application WO 99/28623.Such a device does not provide an optimum transfer of energy from thepassing waves to the converter.

Therefore is therefore a need for an improved wave energy conversiondevice.

OBJECT OF THE INVENTION

It is an object of this present invention to provide an improved devicefor extracting energy from waves or a swell in a body of liquid.

SUMMARY OF THE INVENTION

Accordingly the invention provides a wave energy conversion apparatusfor harnessing energy from wave motion comprising:

-   -   at least two devices, each device comprising a surface float        and/or at least one submerged body below the surface,    -   linkages between the at least two devices, and        wherein the at least two devices are adapted to move relative to        one another in response to passing waves or swell in the body of        liquid, and which relative movement between the at least two        devices may be harnessed by the linkages between the at least        two vessels or devices.

Desirably each of at least two devices comprises a surface float rigidlyconnected to at least one submerged body below the surface float.

The movement between the at least two devices preferably effects anenergy generation which is harnessed by the linkages.

By the term rigidly connected is meant that the connection between thesurface float and the at least one submerged body is sufficiently rigidto transmit tension and compression forces.

The at least one submerged body is preferably submerged at a depth belowthe surface that is a significant fraction of the length of theprevailing wavelengths of wave or swell in the body of liquid.

The at least one submerged body is preferably adapted to entrap volumesof the surrounding liquid or may alternatively or also in part entrap orcontain airspaces or buoyancy devices.

The surface floats are preferably of a size and weight sufficient toensure that they remain partially submerged in the water under normalwave or swell conditions.

By the term surface float is meant a surface-piercing body normally atleast partially submerged, wherein at least part of the float normallypierces or projects above the level of the fluid in which the float ispresent.

By normal conditions is meant conditions that are typical for prevailingweather conditions and wave/swell size in the area of deployment of theapparatus.

Preferably each device comprising a surface float, submerged body orvessel and entrapped liquid has an overall mass, virtual mass anddimensions such that it will either tend to have a natural frequency ofoscillation along its vertical axis that is close to the dominantfrequency of the surface wave or have a means to alter its naturalfrequency to match that of the prevailing wave climate.

By virtual mass is meant minimal gravitational weight and refers to abody that provides high hydrodynamic ballast or inertial mass with whichthere will be ‘added mass’ associated with its movement through thefluid. This desirably may be an enclosed vessel entrapping the liquid inwhich it is immersed, or alternatively a partially enclosed vessel or asubmerged horizontal flat plate. The virtual mass is intended to providea resistance to the acceleration of the rigidly linked surface float. Itwould preferably be smooth and streamlined to reduce drag and mayenclose buoyancy to minimise gravitational weight.

The apparatus may additionally comprise adjustment means by which theentrapped volume of the submerged vessels may be adjusted by, forexample, adjusting the volume of liquid entrapped in the submergedvessel(s).

The linkages are adapted to operate a power take off system which maytypically be hydraulic and driving an electric alternator. The linkagesare preferably arranged to allow several degrees of freedom of movement,and thus to collect additional power from relative movements arisingfrom pitching and rolling as well as from vertical heave.

Preferably, the apparatus includes tuning and control systems linked toprobes or detectors or an operating console either onboard and/or remotefrom the at least two devices.

The apparatus may additionally comprise latching and/or phase controlmeans adapted to assist and optimise the oscillation, amplitude andrelative motion of linked devices in varying or heavy sea conditions.This may be effected by using hydraulics or air springs to freeze ortemporarily dampen the movements of the devices and or to return powerto the apparatus at certain stages in the cycle.

The apparatus may also include mooring systems that maintain thecomplete apparatus in a position that is consistent with statutoryrequirements and not significantly inhibit its efficient operation.

In addition to the aforementioned power take-off linkages, linkeddevices may also have elastic links or chains or shock absorbers orsimilar adaptations to absorb excessive relative movements in heave orsurge or pitch that may be caused by breaking seas in storm conditions;such elastic links or chains would normally be slack and may beweighted.

Other advantages and features of the present invention will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the principle of operation of an apparatusof the present invention under wave conditions, with the floats andassociated submerged bodies oscillating out of phase with one another,

FIG. 2 is a schematic showing one possible arrangement suitable foradjusting the virtual mass of the submerged body of the device of thepresent invention,

FIG. 3 is a perspective view of a practical embodiment of the presentinvention,

FIG. 4 is a side view of the embodiment illustrated in FIG. 3,

FIG. 5 is a plan section along the line A—A of FIG. 4,

FIG. 6 is a schematic showing a possible arrangement for the powertake-off linkages, illustrating possible degrees of freedom in x, y, zand one of rotation,

FIG. 7 is a schematic illustrating a power take off circuit for use withthe present invention, and

FIG. 8 is a schematic illustrating an alternative embodiment of theapparatus of the present invention equating to a conventional pointabsorber but incorporating a virtual mass.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show an apparatus 1 for harnessing energy from waves orswell in a body of liquid 2 in accordance with one embodiment of thepresent invention. It comprises two devices: an inner device 3, and anouter device 4. Both the inner and outer devices comprise surface floats5, 6 rigidly connected by means of stiff rods 7, 8 or other suitablemeans, to respective submerged bodies 9, 10 below the surface. Thesurface floats 5, 6, may preferably incorporate watertight bulkheads.The submerged bodies 9, 10 are preferably adapted to entrap volumes ofthe surrounding liquid or may alternatively or also in part entrap orcontain airspace's or buoyancy devices. The submerged bodies 9, 10 maybe considered to have a virtual mass: by the term virtual mass is meantminimal gravitational weight and refers to a body that provides inertialmass and hydrodynamic ballast or added mass. This may preferably be anenclosed vessel, but also suitably a partially enclosed vessel or asubmerged horizontal flat plate or some combination of these. Thevirtual mass is intended to provide a resistance to the acceleration ofthe linked surface float.

In the view shown in FIG. 1 the surface level 11 is not flat, i.e.comparable to a passing swell, and the two devices are adapted to moverelative to one another in response to the passing waves or swell in thebody of liquid. Preferably, the overall mass, virtual mass anddimensions of each device comprising surface float, submerged body orvessel(s) and entrapped liquid is such that each will tend to have anatural frequency of oscillation along its vertical axis that is closeto the lower end of the range of frequencies that corresponds to thebulk of the passing wave energy. The actual frequency of oscillation ofthe linked devices may be increased above their natural frequency bymeans of phase control systems, for example by hydraulic means, so thatthe apparatus tends to move in phase with the immediate wave climate.The natural frequencies of the two linked devices may be furtheradjusted by altering the amount of the respective virtual mass (i.e.inertial mass and added mass) to which each is attached. These virtualmasses resist the acceleration and hence rate of response of the linkedfloating body to the driving forces caused by the passing wave in bothrising and sinking motions. By altering these virtual masses it ispossible to arrange that the two linked devices tend to oscillate withdiffering phase angles. The amount of difference may typically be aquarter of the frequency of the dominant wave. This relative movementbetween the two devices 3, 4 effects an energy transfer which may beharnessed by linkages 12 between the two devices 3, 4.

It will be appreciated by those skilled in the art that any combinationor number of submerged bodies may be utilised.

As shown in FIG. 2, using the example of one submerged body 9, thesubmerged bodies 9, 10 may additionally comprise adjustment means bywhich the inertial mass of the submerged vessels may be adjusted by, forexample, adjusting the volume of liquid entrapped in the submergedvessel(s). The submerged body may include a cylindrical compartment 13running the length of the submerged vessel, but sealed off from theremaining portion of the sealed vessel. When open at both ends thecylindrical compartment will “hold” no liquid, and as such can beconsidered to have no inertial mass. By activation of a valve 14, whichmay be at the top and/or bottom of the cylindrical vessel or above bymeans of a narrow bore extension open to the atmosphere via surfacefloat it is possible to vary the entrapped mass contained within thecompartment 13. The submerged body may have a plurality or none of suchcompartments contained therein. This method of adjustment can have theadditional benefit of adjusting the added mass associated with thesubmerged body by effectively altering its cross-sectional area in thedirection of the heaving movement.

FIGS. 3 to 5 are illustrations of a practical embodiment of theapparatus illustrated in FIG. 1 except that the power take off linkagesand covering superstructures are omitted, and a sea-bed 100 is includedfor reference purposes. The same reference numerals will be used forsimilar components. FIG. 3 includes a person 14 identified for scalepurposes. Due to the large dimensions of the apparatus and the typicaldeployment in ocean conditions it may additionally comprise navigationlights and radar reflectors 15 to identify it to shipping As shown inthe side view of FIG. 4 and the section of FIG. 5, the outer devicecomprises a float 6 linkable to four submerged bodies 10, which in thisembodiment are arranged concentrically about a submerged body 9 which ispart of the inner device 5.

FIG. 6 is a schematic of a view of an apparatus of the present inventionabove the surface of the sea. The inner 5 and outer 6 surface floats areevident, as are the power take off linkages 12, maintaining thecommunication between the inner and outer devices 3, 4. In thisillustrated embodiment the linkages 12 incorporate hydraulic cylinders30 and are connectable to a central shaft 31 rigidly connected to theinner oscillating unit. This arrangement of pistons will allow threedegrees of freedom of movement between the two oscillating devices 5, 6;power may be collected, via movements in the hydraulic cylinders 30,from pitching and rolling as well as from vertical heaving. A furtherdegree of freedom is allowed by the pistons being connected to thecentral shaft 31 via a rotatable collar member 20, which is adapted toallow the outer device to rotate completely about the inner device.Although such rotation does not actively effect changes in energy itenhances the seaworthiness of the entire apparatus in conditions whererelative slewing of the connected devices may arise. The freedom torotate ensures that such slewing will not disable or break the linkagesbetween the inner and outer devices. Although described with referenceto a device wherein the linkages 12 are positioned above the surfacefloats 5, 6 it will be appreciated by the skilled person that there aresuitable alternative arrangements that will allow power to be collectedfrom a relative movement between two or more bodies, and allows degreesof freedom between the two or more bodies.

The hydraulic linkages 12 of the present invention are typically of thetype known in the art as heave compensators or double acting actuators.The incorporation of such actuators into a power take-off arrangement isillustrated in FIG. 7, which describes a system for the conversion ofthe relative movement of the two devices into electricity. It will beappreciated by those skilled in the art that this power take-offarrangement is illustrative of the type that may be used and is notintended to restrict the invention to such an arrangement. As shown inFIG. 7, two or more actuators 30 are mechanically linked to apressurised hydraulic accumulator or reservoir 41, and the movement ofpistons 42 through the actuator drives fluid from the reservoir 41through flow turbines 33 to drive a hydraulic gear pump 34. This in turnis adapted to power an electric alternator 35. By provision of a flowcomputer 36, or other suitable arrangement, it is possible to controlthe generation of power from the actuators 30.

Heretofore the invention has been described by means of an apparatuscomprising two point absorber devices each capable of independentoscillation. FIG. 8 illustrates an alternative embodiment thatcorresponds to a single point absorber with a moving float 43 reactingagainst a deeply submerged virtual mass 44.

Mode of Operation

As described above, the apparatus of the present invention preferablyderives its power from the relative motion of the two (or more) devices3, 4 each comprising a float of fixed buoyancy 5, 6 on the surface 11 ofthe liquid 2 rigidly connected to one or more deeply submerged rigidvessels 9, 10.

Each surface float 5, 6 tends to act as a heaving buoy as surface wavespass under it and its vertical rising and sinking movements are impededby it being rigidly connected to one or more deeply submerged vessels 9,10 of substantial virtual mass. Each combination of surface float plusattached submerged vessel, as a vertically floating structure, will haveits own natural frequency of oscillation along its vertical axis andwhich can be adjusted by appropriate design and control; each devicebeing stable about a vertical position.

Each combination of surface float plus submerged vessel(s) is driven byforces caused by the passing waves. These excitations force consist of:

-   -   (a) a hydrostatic term (proportional to the instant value of the        wave height)    -   (b) a dynamic component (proportional to the instant liquid        acceleration within the wave)    -   (c) a diffraction force.

The latter two act to reduce the magnitude of the excitation force; Theeffect of the dynamic component is proportional to the volume of thesubmerged body 9, 10 and the submerged part of the surface float;because this effect diminishes with depth it is desirable therefore toplace the submerged body 9, 10 at a depth sufficient to optimise designand efficiency of operation. The diffraction force is a function of theadded mass of the submerged part of the surface float 5, 6. Therefore,in ideal situations, it is desirable to minimise the draught of thesubmerged float 5, 6, i.e. to minimise the dead weight of eachcombination of float plus submerged vessel.

The device as a whole will typically also lose energy because of:

-   -   (d) radiated waves;    -   (e) hydraulic drag;    -   (f) effects of moorings and umbilicals.

The negative effect of (d) and (e) may be minimised by appropriatehydrodynamic design; the effect is (f) is, desirably small with respectto the main excitation forces and is in turn minimised by good designand small overall aspect to lateral forces from wind and wave action andcurrents. Slack or buoyed reduced weight moorings may be appropriate.

Suitably, by appropriate sizing and design, the natural frequency alongthe vertical axis of each combination of float and submerged vessel(s)may be designed to be close to that of the dominant wave frequency, thusproviding the best chance for resonance to occur in the absence of aphase control system.

The control systems may be optimised to enable the oscillations of theapparatus to be adjusted to match the prevailing wave period so thatresonance may occur over a range of wave periods. In order to effectthis, it will be necessary to design the basic apparatus such that itsnatural frequency will be higher (or its period shorter) than that ofthe majority of the waves; the control systems may then be used to slowthis frequency, so that the phase velocity of the apparatus and that ofthe wave are matched and that the amplitude of oscillation tends towardsan optimum for maximum useful power for the conditions.

In heavy seas, but not necessarily destructively extreme seas, suchphase control and or latching will enable useful power to be recoveredand efficient performance maintained even though the amplitude ofoscillation would otherwise exceed the design limits of the hydraulics(‘stroke out’).

An apparatus designed to have a natural frequency at the lower end of aselected range of wave periods and substantially less that the mostcommon period means that that apparatus will be smaller than onedesigned to match the most common period. This has the additionalbenefits of lowering the capital cost of the apparatus and of itsmoorings and also of reducing the risk of loss in extreme seas.

It is also possible to adjust the natural frequency of each combinationof surface, float and submerged vessel(s) by adjusting the volume offluid entrapped by the submerged vessel(s) by, for example, causingvalves to open at the top and bottom of narrow vertical chambers withineach or any one submerged vessel.

In order to implement a suitable control system it is necessary to beable to predict and take appropriate action for the immediatelyanticipated wave and also to adjust the device to take account oflonger-term trends. A wave prediction model may be incorporated in thesoftware as will a memory function and data logging. The control systemparameters may be altered remotely, including adjustments based onhistorical performance, weather forecasts, remotely sensed data, andstorm alerts.

Thus the relative motion of the two devices 3, 4 each comprising surfacefloat 5, 6 and submerged vessel(s) 9, 10 and a phase control system maybe adjusted to be close to a resonance condition over a selected rangeof wave periods. The magnitude of oscillation tends to a maximum closeto resonance; the preferred embodiment of the device takes advantage ofthe rapid change in phase shift close to resonance when adjustingparameters of the oscillating bodies. By this tactic a pair of devices,each comprising float and submerged vessel(s), may have a significantdifference in phase shift with respect to that of the incident.

The difference in phase shift between a pair of linked devices may beexploited as a source of power by means of some suitable system ofmechanical links or by electrical induction. It is to be noted that theamplitude of the relative motion between the two devices is at all timesless than the amplitude that might be expected from a single deviceeither following a wave or in resonance with a series of waves. Thisfeature greatly reduces the incidence of amplitude exceeding the designlimits of the hydraulic cylinders (‘stroke out’) and facilitates the useof hydraulic cylinders of shorter stroke and hence lower capital costs.

Theoretical analysis has shown that this apparatus, effectively acombination of two oscillating devices each a point absorber, is capableof absorbing significantly more power from a passing wave than can asingle point absorber. In order to achieve these higher efficiencies itis necessary to carefully select the correct proportions for the surfacefloats and their respective draughts.

In the alternative form of the apparatus, illustrated in FIG. 8, theapparatus comprises a single surface-piercing float and a deeplysuspended virtual mass linked by means of the power take-off similar tothat already described. In this embodiment the amount of power that maybe recovered from the passing waves may approach the theoretical limitfor a simple point absorber, i.e. from the wavelength divided by 2π.However the incorporation of a large virtual mass, a feature of thispresent invention, facilitates the design of a simple and low-costapparatus with a large installed capacity well matched to long-wavelength and powerful ocean swell and suitable for offshore conditions.This embodiment has the additional advantage of allowing a greateramount of power to be absorbed from the pitching and lateral motion ofthe surface float as it is not inhibited by rigid attachment to a deeplysuspended vessel.

Preferably said means for converting resulting forces or changes inforces in the apparatus to useful energy may be selected from one ormore of the following:

-   (a) a hydraulic system-   (b) a pneumatic system-   (c) a mechanical system-   (d) a piezoelectric system-   (e) an electrical system

Preferably said means for converting said forces or changing forces inthe apparatus converts said change to an output device which is selectedfrom one or more of the following:

-   (a) an electricity generating device-   (b) a device for the hydrolysis of water-   (c) a pumping device-   (d) a device for making potable water-   (e) a device for extracting dissolved salts-   (f) a hydraulic device-   (g) a mechanical device

The output device preferably generates power in a cyclical manner andthe apparatus may optionally further comprise means whereby power can betaken out of tie system during one part of a cycle and put back into thesystem during another part of the cycle.

The combination of two or more converters in an array will provideopportunity to share costs associated with the power take off systemsand also improvements in the continuity and supply of power.

In any of the aforementioned described embodiments the hydrauliccylinder, accumulator and motor generator may be accommodated in an‘engine room’, preferentially capable of being detached for maintenancepurposes.

The device, in any marine embodiment, is intended to be located awayfrom the shoreline and outside the zone of impacting breaking waves.This will result in a more constant generation of power than otherdevices. The floating vessels 5, 6 will preferably be hermeticallysealed, partitioned internally, and will have minimal resistance tobreaking seas or very large waves. It may readily be designed such thatexceptionally steep waves or breaking seas will pass over it, a form ofhydrostatic clipping.

With phase control systems, wave prediction, and being axi-symmetric,the wave energy converter 1 will continue to perform effectively inirregular seas, a condition that is more usual than regularmonochromatic wave forms. Very frequently the waves are the result oftwo or more patterns superimposed, with perhaps an underlying longwavelength swell where the chosen site is open to the ocean andprevailing wind directions. The design imperative is to obtain usefulpower at low cost, i.e. to optimise the unit cost of power delivered,rather than to seek to achieve the conversion of the maximum amount ofthe available wave power.

The apparatus of the present invention utilises simple and robustcomponents and systems. As such it is possible to maximise availabilityand simplify maintenance of deployed power conversion devices, which maydeployed singularly or in large arrays. These arrays may be arrays ofindividually moored wave energy converters, which is typically thepreferred arrangement for seas with predominantly long wavelengths.Alternatively, the oscillating unit of float and rigidly linkedsubmerged virtual mass may be deployed in a floating and rigid openframework of adjoining cells, an arrangement that may suit shallower andmore sheltered seas with generally shorter wavelengths. Sucharrangements in arrays may allow the sharing of a common functionalitybetween several devices.

The devices are designed be independent of tidal changes in mean sealevels, have minimum dependence on wave direction, and maximise thereturn from long wavelength ocean swell.

By utilising the advantages offered by totally or partially submergeddevices it is possible to move out beyond the shoreline and the breakingwave zone and generate of the order of 0.5 MW to 1 MW or more per devicein a suitable wave climate.

This approach represents a highly efficient converter of wave energy andimprovement on previous devices in that:

-   the use of submerged vessels having inertial mass as high as desired    and gravitational mass (net buoyancy) as low as desired permits the    self-reacting ‘heaving buoy’ point absorber to approach an ideal    mass-spring arrangement and to be far more powerful than was    hitherto considered possible-   the use of streamlined inertial masses avoids inefficiencies and    energy losses due to drag and vortex shedding associated with    inertial plates either alone or in accelerator tubes.-   by extracting power from the relative motion of two coupled devices    the stroke length is much reduced, even at resonance, and hence the    losses associated with stroke-out reduced and or larger seas may    usefully be exploited-   phase control, either continuous or intermittent (‘latching’),    allows the use of smaller devices operating close to resonance    across a range of wave periods.-   all submerged elements are simple fabricated components,-   power take-off and control apparatus may be located within the    surface floats,-   the apparatus may be fabricated in a dry dock and towed to the    selected site,-   the apparatus is floating and self-reacting and independent of tidal    differences,-   it is well suited to the demanding offshore conditions.

The combination of a submerged variable buoyancy and a deeply suspendedgravitational mass, as in our corresponding international application WO99/28623 could be considered as a single very large virtual mass withhigh gravitational mass, but with little or almost neutral buoyancyeffected by an associated submerged buoyancy. In this way an idealmass-spring arrangement is provided, something that had heretofore notbeen realised as possible for a floating wave energy converter. Bysuitable design of the submerged components any losses due to drag maybe minimised. In this present invention, this combination of submergedbuoyancy and large inertial mass is replaced by a closed vessel of smallintrinsic mass, but entrapping a large and therefore massive volume ofwater, thereby acting as a large inertial mass. The combination ofsurface float and large deeply suspended virtual mass may be tuned tothe prevailing wave climate, facilitating resonance across a range ofwave periods, an important property if maximum power absorption is to beachieved. The use of hydraulic power take-off systems facilitates theincorporation of suitable forms of phase control.

By closely combining two such devices separately tuned, a relativelylarge phase shift may be achieved.

Further improvements are effected by ensuring that all the submergedvessels and assemblies are streamlined and finished to minimise drag andthe spaces between the two or more oscillating and connected devices iskept sufficiently great to make shear forces insignificant (a gap ofabout 1 metre in ocean conditions).

This solution is unlike any previously described self-reacting heavingbuoy point absorber in two ways,

-   it incorporates a relatively very large inertial and associated    added mass as a single virtual mass to react against, and secondly-   it is preferably a combination of two oscillators that may be    differentially tuned to allow maximum phase shift in all of which    power is taken off from their relative movement.    It is further enhanced by being capable of collecting useful power    from relative motions between the connected devices other than the    vertical mode.

1. A point absorber wave energy conversion apparatus (1) for harnessingenergy from wave motion on the surface of a body of liquid (2) andhaving dimensions small with respect to the wavelength of thepredominant wave, the apparatus comprising: a) at least two devices (3,4, 43, 44), each device comprising a surface float (5, 6, 43) and atleast one submerged body (9, 10, 44) below the surface of the body ofliquid, b) linkages (12) between the at least two devices, wherein theat least two devices are adapted to move relative to one another inresponse to passing waves, and the relative movement between the atleast two devices effects an energy transfer which may be harnessed bythe linkages between the at least two devices.
 2. The apparatus asclaimed in claim 1 wherein each device (3, 4) comprises a surface float(5, 6) rigidly connected to at least one submerged body (9, 10) belowthe surface float.
 3. The apparatus as claimed in claim 1 wherein the atleast one submerged body is submerged at a depth below the surface thatis a significant fraction of the length of the prevailing wavelengths ofwave or swell in the body of liquid.
 4. The apparatus as claimed inclaim 1 wherein the at least one submerged body is adapted to entrapvolumes of the surrounding liquid or may alternatively or also in partentrap or contain airspaces or buoyancy devices.
 5. The apparatus asclaimed in claim 1 wherein the devices are of a size and weightsufficient to ensure that the surface floats remain partially submergedin the water under normal wave or swell conditions.
 6. The apparatus asclaimed in claim 1 wherein the overall mass, virtual mass and dimensionsof each device comprising surface float, submerged body, entrappedliquid and control systems is such that each will tend to have afrequency of oscillation along its vertical axis that is close to theprevailing frequency of the surface wave.
 7. The apparatus as claimed inclaim 1 further comprising adjustment means (13, 14) by which theentrapped volume of the submerged vessels may be adjusted, preferablycomprising means for adjusting the volume of liquid entrapped in thesubmerged vessel.
 8. The apparatus as claimed in claim 6 wherein theapparatus includes control systems and/or control systems linked toprobes or detectors and adapted to tune the oscillating device to matchprevailing wave periods by one or more of the following: a) by adjustingthe virtual masses and/or b) to effect latching or phase controlmechanisms intended to maintain oscillation close to resonance duringchanging conditions and/or c) to hydraulically or otherwise lock orotherwise secure the apparatus during maintenance or storm conditions.9. The apparatus as claimed in claim 1 wherein the linkages are adaptedto allow several degrees of freedom and at the same time collect usefulpower from lateral, pitching and heaving relative movements.
 10. Theapparatus as claimed in claim 1 further comprising mooring systemsadapted to maintain the complete apparatus in a substantially stationaryposition relative to a fixed location.
 11. The apparatus as claimed inclaim 1 wherein at least a portion of the linkages may be formed by anair-spring or mechanical spring enabling the connection between the atleast two bodies to be adjusted in accordance with the oscillation ofeither body.
 12. The apparatus as claimed in claim 1 wherein the atleast one submerged body is streamlined so as to reduce drag.